In order to develop better paramagnetic contrast agents to improve diagnostic capabilities and specificity of MRI, it is important to understand these agents and how they work. The physical characterization of MRI contrast agents by EPR, ENDOR, NMR, and ESE spectroscopy is carried out. Systems studied thus far are Gd(III) based complexes, including those used for human MR imaging. Information being obtained by this approach is clarifying the structure of chelates used to complex Gd(III), as well as the nature of waters of hydration in the first and second hydration spheres. The kinetics of hydration sphere water exchange is also being studied.